Method and apparatus for producing steel

ABSTRACT

A steel rolling mill including a Steckel mill is provided with an in-line upstream quench station located downstream of the caster and upstream of the reheat furnace, a shear located downstream of the Steckel mill, and a temperature reduction station downstream of the shear. The upstream quench station has spray nozzles that quench a surface layer of the steel to transform same from an austentitic to a non-austentitic microstructure. The shear provides a precise transverse vertical face on the leading end of the steel. The temperature reduction station applies cooling fluid to the rolled steel so as to obtain a preferred microstructure that may be either bainite or martensite. If bainite, the temperature reduction station includes laminar-flow cooling apparatus; if martensite, the station also includes an initial rapid quench, in which latter case the station is followed by a tempering furnace.

RELATED APPLICATIONS

[0001] This is a divisional application of (1) U.S. application Ser. No.09/350,314, filed Jul. 9, 1999, which is a continuation-in-part of (2)U.S. application Ser. No. 09/157,075, which is a continuation of U.S.application Ser. No. 08/594,704, now issued as U.S. Pat. No. 5,810,951,and (3) U.S. application Ser. No. 08,870,470, which is a continuation ofU.S. application Ser. No. 08/481,614, now issued as U.S. Pat. No.5,706,688, and (4) U.S. application Ser. No. 09/113,428. Thisapplication also claims priority from U.S. application Ser. No.09/113,428, filed Jul. 10, 1998.

FIELD OF INVENTION

[0002] This invention relates to an apparatus combination in or for usein a steel-making mill, and a preferred method of operating same.

BACKGROUND OF THE INVENTION

[0003] In in-line rolling mills, high speeds of operation are essentialif undue cooling of the steel below optimal rolling temperatures is tobe avoided. Consequently, in such mills, control of both internalmicrostructure and of external surface properties is difficult toachieve to produce optimum metallurgical characteristics in the finishedsteel product.

[0004] In a conventional continuous castir steel mill using a reversingrolling mill such as a Steckel mill for rolling, steel is cast into astrand by a caster, is severe Id into slabs or other preferred portionsdownstream of the caster, and passes to a reheat furnace where it isheated to a uniform pre-rolling temperature. From the reheat furnace,the steel may be rolled by an initial roughing mill, but at least thefinal rolling steps are effected by a Steckel mill. If thicker plateproduct is produced by the Steckel mill, the Steckel mill rolling isconfined to flat-pass rolling. For thinner, coilable plate and stripproduct, the steel may be coiled within the coiler furnace during atleast the later rolling passes. Following rolling, the steel istypically cooled, and if finished as a plate product, is conventionallypassed through a hot leveller or cold leveller or both. Coilableproducts are typically up-coiled or down-coiled and offloaded forshipment; flat plate is cut to preferred length and may be subjected tofinal cooling on a cooling bed before stacking and shipment.

[0005] The use of a Steckel mill to roll steel is well established inindustrial practice and in the technical literature. However, theoptimization of steel quality from both a surface standpoint and aninternal microstructure standpoint, especially for flat steel plateproducts, has not been achieved by others. In particular, it has notbeen understood prior to the development of the present invention by thepresent inventors that a unique combination of steel cooling, steelheating and steel reduction steps, taken in a controlled manner betweenthe caster and the end of the downstream line, can lead to preferredsurface and metallurgical characteristics in finished steel plate.

SUMMARY OF THE INVENTION

[0006] We have found that optimal metallurgical characteristics of thesteel thus produced, and particularly steel plate products thusproduced, may be optimized by a careful selection of the combination ofapparatus that is provided between the caster and the more remotedownstream apparatus, and if appropriate operational constraints areapplied to this combination of equipment.

[0007] Generally speaking, preferred metallurgical results require anoverall reduction in thickness of the steel of at least about 3:1.Consequently, if the objective is to make 1/2″ plate, the initialcasting must be at least 1.5″ thick. Further, for various other reasonsit may be preferred to use castings of greater thickness than threetimes the target end-product thickness. The present invention ispreferably used with castings of at least about 3 inches in thickness.

[0008] Specifically, the apparatus combination protected by thisinvention comprises a quench facility located downstream of the casterand upstream of the reheat furnace (“upstream quench station”), and acontrolled temperature reduction facility located closely downstream ofa Steckel mill or other similar reversing rolling mill. Note that“closely” does not preclude the possible installation of devices betweenthe Steckel mill and temperature reduction facility. A hot flying shearor other suitable severing device is also required; ideally two hotflying shears one upstream and one downstream of the temperaturereduction facility would be used. However, depending upon the intendeduse of the rolling mill, it is possible to make do with one flyingshear. Some preferred uses of the rolling mill favour upstream locationof the flying shear; others favour downstream location. For example,some of the rolling optimization aspects of the invention and theobjective of presenting a clean vertical leading edge of the steel tothe downstream quench station favour an upstream location of the hotflying shear. On the other hand, the need to accelerate a leadingsevered portion of the steel relative to a trailing portion when thesteel is cut to length favours a cut-to-length flying shear downstreamof the temperature reduction facility. The applicable considerationswill be reviewed further later in this specification. To accommodate allpossible objectives, two shears may be provided—one upstream of thetemperature reduction facility, and one downstream. If the mill is toproduce steel of an appreciable range of thicknesses, one type of shearcould be provided for thinner steel, another type for thicker steel.Further, some shears work well only for hot steel, others for cold. Themill designer will take these points into consideration in the overallequipment selection and design.

[0009] The upstream quench station may be located either upstream ordownstream of the slab severing apparatus (typically a cutting torch)This upstream quench station is constrained to apply to the steel acontrolled surface quench with a depth of penetration of at least{fraction (1/2)} inch, and preferably no more than about {fraction(3/4)} inch (while quench penetration deeper than {fraction (3/4)} inchis metallurgically tolerable, it conveys no additional benefit so far asthe steel surface is concerned, and the greater the depth of penetrationof the quench, the greater the amount, of heat required in the reheatfurnace to heat the steel to uniform pre-rolling temperature, and thusthe higher the cost of production). The quench imparted to the steelmust be sufficient to alter the surface layer of the steel fromaustenite to some other microstructure such as ferrite or pearlite.Preferably the quench is initiated when the surface temperature is at orabove the austenite transformation start temperature Ar₃, although starttemperatures somewhat below the Ar₃ can be tolerated, even though suchlower temperatures are not optimum. A reduction in temperature by thequench station in the order of about 250-300° C. is preferably effected.Assuming a preferred start temperature at or above the steel'stransformation start temperature Ar₃, a suitable completion temperatureis at or below the steel's transformation completion temperature Ar_(1.)

[0010] The steel transformation start and completion temperatures Ar₃,Ar₁ depend on the type of steel that is cast and the cooling rate. Mosttypes of steel cast in a conventional continuous casting mill aresuitable for application of the invention; for example, typical plaincarbon steels suitable for quenching in accordance with the inventioninclude steels having 0.03-0.2 Σ carbon content. The cooling rate of asteel product is not constant throughout its body; cooling rates differat different depths beneath the product surface. Different cooling rateswill transform austenite to different combinations of transformationproducts; as the steel's cooling rate varies with strand depth, itfollows that the transformed microstructure will differ with stranddepth.

[0011] For optimal results, the quench should be applied in a mannerthat responds to the surface temperature gradient of the casting, whichtypically is hotter at the inner surface portions near the longitudinalcentre of the casting than at the outer side edges of the casting. Tothis end, a transversely differential spray is arranged within thequench unit. Since spray applied above the casting is typically moreeffective than spray applied underneath the casting, the ratio of bottomspray flow rates to top spray flow rates is preferably in about therange of 1.2 to 1.5. Since the side edges of the casting tend to coolmore rapidly than the central portions, and since there is a tendency ofany accumulation of surface water to flow from the central portion overthe side edges, no spray may be required for the side edges, andfurther, the side edges may be protected against overcooling. Suchprotection may include longitudinally extending suction devicesoverlying the side edges (adjustable for width of casting) and maskingof the side edges to impede cooling spray.

[0012] Further line control of the quench spray may be provided toaccommodate changes in casting speed, and other variations that couldresult in non-uniform quenching of upper and lower surfaces of thecasting.

[0013] The upstream quench station facilitates the production of platehaving a surface relatively free of defects.

[0014] In the reheat furnace, the steel is reheated to a uniformpre-rolling temperature suitably above the austenite transformationtemperature T_(nr). Rolling in a Steckel mill proceeds with theinevitable pauses between passes to permit the steel to decelerate andthe Steckel mill to reverse the direction of rolling pass and toaccelerate the steel for the next following reduction. These pausespermit a greater opportunity for controlled recrystallization of thesteel to occur while the temperature of the steel is above the T_(nr)than is available for in-line rolling through a series of sequentialroll stands. Preferably, any given portion of the steel is at “rest”(not subjected to a reduction operation) for a cumulative total “rest”time of at least about 60 seconds during the rolling procedure so as tooptimize the controlled recrystallization (by “rest” is not meant thatthe steel is not moving longitudinally; by “rest” is meant that thesteel is not being actively reduced by the Steckel mill rolls). If thesteel product is, a coilable product, some rolling above the T_(nr) maybe effected at steel thicknesses below the minimum coilable thickness ofthe steel while using the Steckel mill coiler furnaces for winding ofthe steel between passes, and the heat within the coiler furnace mayimpede temperature drop of the steel below the T_(nr), thus makingpossible a greater amount: of time during which controlledrecrystallization can occur than would be the case if the coilerfurnaces cannot be used. On the other hand, steel that is to be rolledto thicker end products tends to retain heat to a greater extent thanthinner steel products, and consequently may be flat-passed rolled for asufficient number of passes above the T_(nr) that an adequate amount ofcontrolled recrystallization can occur.

[0015] For optimum metallurgical results, the steel is rolled above theT_(nr) for a selected number of rolling passes to achieve a reduction ofthe steel of at least about 1.5:1. Thereafter, the steel is rolled belowthe T_(nr) for a further selected number of rolling passes so as toachieve a further reduction of the steel of the order of 2:1. Thecombined effect of the first and second reductions is, therefore, anoverall reduction of at least about 3:1, which is considered to be theappropriate minimum for the obtention of preferred metallurgicalresults. The second reduction is preferably completed at an exittemperature from the rolling mill at about the Ar₃.

[0016] During the reduction rolling below the T_(nr), the fine-grainedaustenitic mircrostructure that was obtained by controlledrecrystaillzation is pancaked. The eventual microstructure desired inthe eventual steel product will vary considerably depending upon theexpected end use of the product. Such preferred microstructure isachieved by a controlled cooling of the steel after the last reductionpass through the Steckel mill.

[0017] The controlled cooling should be effected so that upper and lowersurfaces of the steel are subjected to the initial coolingsimultaneously and uniformly. To this end, a hot-flying shear orequivalent transverse shearing device should provide a leading edge ofthe steel to be cooled that is precisely transverse, planar and verticalwithin engineering limits. As previously noted, this objective is servedby locating a hot flying shear between the Steckel mill and thetemperature reduction facility.

[0018] The nature of the controlled cooling is selected to meet themetallurgical objectives for the end product to be produced. Twodifferent end products will be discussed in this specification by way ofexample.

[0019] A first suitable choice of end product is one containing a highproportion of fine-grained bainite. Such steels have a good combinationof strength, toughness and ductility. To this end, immediatelydownstream of the hot-flying shear, or equivalent severing device, is ancontrolled cooling station that facilitates production of thehigh-bainite-content product.

[0020] The steel, at about the Ar₃ temperature, is subjected in thecontrolled cooling station to controlled cooling of about 12 C to about20 C per second, and preferably about 15 C per second, so as to reducethe temperature of the steel by at least about 200 C and preferably atleast about 250 C. Since the Ar₃ for most commercial grades of steel ofinterest is typically of the order of 800 C or at least in the range ofabout 750-800 C, it follows that the exit temperature from the coolingstation will be no higher than 600 C and typically no lower than about450 C, land most probably and preferably in the range of about 470 C toabout 570 C. The temperature drop imparted by the controlled cooling canbe more than 250 C below the Ar₃, but should riot be more than about 400C below the Ar₃ and preferably in the range about 250 C to about 350 Cbelow the Ar₃.

[0021] The controlled cooling station is preferably laminar flow coolingapparatus so far as the upper surface of the steel being processed isconcerned; the undersurface of the steel product is preferably cooled bya quasi-laminar spray. The usual spray medium is water, maintainedwithin conventional temperature ranges.

[0022] The amount of the temperature drop from the Ar₃ imparted by thecontrolled cooling will depend upon the chemistry (alloy composition) ofthe steel being rolled, in the discretion of the metallurgist who isresponsible for the steel processing.

[0023] Another example of a suitable steel product to be produced isplate having a high proportion of fine-grained martensite. In such case,the controlled cooling facility downstream of the hot-flying shear, orequivalent device, would be a quench station (“downstream quenchstation”) followed by a laminar-flow cooling facility, and in turnfollowed by a tempering furnace off-line. The downstream quench stationwould impart an initial severe quench to this steel, the laminar-flowcooling to follow would maintain cooling of the steel at a ratepreferably equal to the maximum rate permitted by the heat-transfercharacteristics of the steel.

[0024] More particularly, after being cut by the hot flying shear, thesteel is passed through and is rapidly cooled by a roller pressurequench (RPQ) apparatus, thereby transforming the surface layers of theproduct into martensite. As a result of the quenching, the product'ssurface is chilled to about the temperature of the applied coolingfluid. The product then passes through and is further cooled in ancontrolled cooling station. The controlled cooling maintains thetemperature of the chilled surface layer, thereby provided a maximumtemperature gradient between the surface and the product core, in turnenabling a maximum rate of heat dissipation out of the core. The rate ofheat dissipation and the temperature of product after controlled coolingexceeds the critical martensite cooling rate, and the temperature withinthe steel falls below the martensite start temperature throughout mostit not all of the product, thereby transforming as much martensite asthe chemistry and cooling rate will permit. Optionally, the RPQ quenchunit may be modified to provide tensioning of the steel between itsinput and output rolls (in a manner similar to that providedconventionally in some types of hot levelling apparatus) to promoteflatness of the steel and possibly to improve its surface quality.Optionally the RPQ quench unit may be provided with suction devices inthe vicinity of the spray nozzle orifices to remove heated water fromthe surface of the steel.

[0025] Note that production of bainite-rich or martensite-rich steelsboth require a laminar-flow cooling or equivalent controlled coolingfacility; the difference is that production of the martensite-rich steelrequires also a quench station and an off-line tempering furnace.Accordingly, the steel mill may be arranged to provide the facilityrequired for martensite-rich steel production, and when the millproduces bainite-rich steel, the quench station downstream of theSteckel mill will be idle and only the laminar flow controlled coolingstation will be used. The laminar flow cooling facility or equivalentwill have to accommodate production of both types of steel it both areto be produced by the mill; to this end, the controlled cooling facilityshould be designed to provide maximum flow to meet peak productionrequirements, with the availability of reduced flow or of idling somebanks of nozzles if maximum flow is not required. The tempering furnace,being off-line, would in any case not interfere with continued on-lineprocessing of bainite-rich steel product If tempering of martensite-richsteel plate is to be effected, the product is after quenching andcooling in the controlled cooling facility optionally hot levelled in ahot leveller, in essentially the same manner as a bainite-rich product.Then, the product in accordance with conventional practice passes to atransfer table and thence transversely to a cooling bed. Then, theproduct is taken off-line and transferred into in a tempering furnaceand heated for a suitable tempering period and at a suitable temperingtemperature. The temperature allows the reconstitution of entrappedcarbon, thereby increasing the ductility of the steel. After temperingthe martensite-rich steel possesses high strength and hardness typicallycharacteristic of quench-and-tempered steels.

[0026] The resulting plate steel product produced by apparatus accordingto the invention is of a preferred fine-grained microstructure whosecharacter will depend upon the nature of the controlled coolingdownstream of the hot-flying shear. The steel product will have asurface relatively tree of defects by reason of the quench impartedimmediately downstream of the caster. The product will have afine-grained microstructure by reason of the controlledrecrystallization that occurs during the initial flat-pass rolling ofthe steel above the T_(nr). This preferred combination of metallurgicalcharacteristics is obtainable in an optimally economical manner by theapparatus combination of the present invention.

THE DRAWINGS

[0027] A detailed description of the preferred embodiment is providedherein below with reference to the accompanying drawings, in which:

[0028]FIG. 1 is a schematic perspective diagram of a steel casting androlling mill incorporating a caster assembly, upstream quench station,reheat furnace, Steckel Mill, hot flying shear, downstream quenchstation, controlled cooling station, and tempering furnace in accordancewith principles of the present invention;

[0029]FIG. 2 is a schematic interior side elevation fragment view of anembodiment of the upstream quench station according to the invention.

[0030]FIG. 3 is schematic plan view of an array of bottom transverselyvariable spray nozzles suitable for use with the upstream quench stationof FIG. 2, and associated fluid supplies thereof.

[0031]FIG. 4 is a schematic diagram of a control unit for thetransmission of air and water to spray nozzles in the array of FIG. 3shown as a fragmentary group.

[0032]FIG. 5 is schematic interior elevation view of top and bottomgroups of spray nozzles within an upstream upstream quench stationaccording to an embodiment of the invention that provides bothtransverse and longitudinal adjustment of flow rate of cooling fluidfrom the nozzles.

[0033]FIG. 6 is schematic plan view of an array of longitudinallyadjustable nozzles and transversely adjustable nozzles and supply linestherefor, for use within an upstream quench station according to anembodiment of the invention that provides both transverse andlongitudinal adjustment of flow rate of cooling fluid from the nozzles.

[0034]FIG. 7 is a schematic diagram showing in greater detail of aportion of the Steckel mill and associated coiler furnaces of FIG. 1;

[0035]FIG. 8 is a schematic diagram showing in detail the Steckel Mill,flying shear, downstream quench station, and controlled cooling stationof the rolling mill in FIG. 1;

[0036]FIG. 9 is a schematic diagram showing in: greater detail of aportion of the downstream quench station of FIG. 1; and

[0037]FIG. 10 is a flowchart indicating a preferred sequence ofoperations for optimizing the efficiency of a rolling mill in accordancewith the principles of the present invention.

DETAILED DESCRIPTION WIT REFERENCE TO ACCOMPANYING DRAWINGS

[0038] Referring to FIG. 1, molten steel is supplied to a caster 11 thatcasts molten steel into a cast steel strand 12. The strand 12 exits thecaster 11 and enters a strand containment and redirection apparatus 16wherein it forms a solidified thin skin, moves from a generally verticalposition to a generally horizontal position, and is straightened. Thedevices just described collectively constitute a caster assembly 21.

[0039] Referring to FIG. 2, After exiting the strand containment andredirection apparatus 16, the strand 12 is fed by a series of rollers 22into a upstream quench station 14 located closely downstream and in-lineto the caster 11.

[0040] The upstream quench station 14 has a housing 113 surrounding thestrand 12. Selected portions of the strand 12 are quenched by aplurality of intense sprays of water and air combined into an air mistapplied by clusters of top spray 10 nozzles 131 and bottom spray nozzles124. (Air mist tends to be more efficient than water to quench steel;however, a water-only spray may be suitable but not preferred). As aresult of the quench, the steel is rapidly cooled from its pre-quenchstart temperature to a suitable completion temperature so that thesteel's microstructure is changed from austenite to one or more suitablemicroconstituents, such as ferrite or pearlite. It has been found thateffecting a surface quench to a suitable depth, then reheating the steelin a reheat furnace 15 downstream of a severing apparatus 13, reduces orprevents altogether the occurrence of surface defects in the steelproduct.

[0041] Suitable transformed microstructures include pearlite, bainite,martensite and ferrite, or some combination of two or more of these. Thepreferred start temperature is at or 25 above the steel's transformationstart temperature Ar₃ and the suitable completion temperature is at orbelow the steel's transformation completion temperature Ar₁. It has beenfound that quenching from a start temperature below the transformationstart temperature Ar₃ and above the transformation completiontemperature Ar₁ is in some cases acceptable but not preferred, asquenching in this temperature range provides some but not as muchreduction in the occurrence of surface defects as quenching from atemperature above the transformation start temperature Ar₃.

[0042] The steel transformation start and completion temperatures Ar₃,Ar₁ depend on the type of steel that is cast and the cooling rate. Mosttypes of steel cast in a conventional continuous casting mill aresuitable for application of the invention; for example, typical plaincarbon steels suitable for quenching in accordance with the inventioninclude steels having 0.03-0.2% carbon content. The cooling rate of asteel product is not constant throughout its body; cooling rates differat different depths beneath the product surface. Different cooling rateswill transform austenite to different combinations of transformationproducts; as the steel's cooling rate varies with strand depth, itfollows that the transformed microstructure will differ with stranddepth. It has been found that a minimum transformed depth of about{fraction (1/2)} to {fraction (3/4)} inch will satisfactororily reducethe occurrence of surface defects.

[0043] The spray nozzle clusters 131, 124 are respectively arranged intoa top array 126 and a bottom array 128, wherein each array 126, 128applies cooling spray to an associated top and bottom surface at thestrand 12. The appropriate proportions of cooling fluid that should beapplied respectively to the top and bottom surfaces so that bothsurfaces are quenched to the same depth can be empirically determined byremoving test portions of the quenched strand and examining theircross-section. The appropriate proportion can then be programmed intothe control system for the quench so that subsequently quenched portionsof the strand will be quenched to the required depth.

[0044] Top and bottom nozzle clusters 124, 131 are arranged inrespective matrix arrays 126, 128 each comprising a plurality of equallyspaced longitudinal banks 130 extending in columns parallel to the lineFIG. 3 illustrates this arrangement for bottom nozzle clusters 124; themirror image of this arrangement would exist for top nozzle clusters 131arranged in banks 130 The number of banks 130 chosen to span thetransverse width of the line depends on the maximum width of the caststrand. In the illustrated embodiment, there are nine banks of bottomnozzle clusters 124 by way of example.

[0045] The maximum number of nozzles 133 in a bank 130 depends on theinterior length of the upstream quench station 14. In the embodimentillustrated in FIGS. 1-3, the length of the upstream quench station 14is limited by the space available between the caster assembly 21 and thesevering apparatus 13. An exemplary eleven nozzles 133 are arrangedalong the length of the upstream quench station 14 for each bank 130.Note that no nozzles 133 are arrayed so as to overlap the conveyor rolls22; although the rolls 2 constitute a direct impediment to nozzleplacement only for the bottom banks 130, the arrangement of the topbanks 130 should mirror that of the bottom banks 130 to ensure spraysymmetry so that uneven quenching of top and bottom surfaces of strand12 is avoided or at least mitigated.

[0046] The bank of nozzles 130 are grouped into four groups 137 a, 137b, 137 c, 137 d. Each group 137 a, etc. comprises at least two banks 130equidistant from the longitudinal center of the line. The center group137 d additionally includes one central bank 130 overlapping the centerof the line. The spray applied to the strand 12 by any group 137 a, etc.(“spray group”) of nozzles 133 is controlled by controlling the flowrate and optionally other usefully controllable characteristics of thesprays (e.g., pressure) of the spray group 137 a, etc. (suchcontrollable characteristics are collectively referred to as “spraycharacteristics”). The spray characteristics of any one spray group 137a, etc. are controllable separately from the spray characteristics ofother spray groups 137 b, etc. as discussed in detail below. Each spraygroup 137 a, 137 b, 137 c, 137 d is supplied water from an associatedrespective water supply pipe 140 a, 140 b, 140 c, 140 d connected to andsupplied by a water pump 144. Each nozzle 133 is provided with air froman air compressor 142 via suitable air supply lines (omitted from FIG. 3for purpose of clarity). The air and water are mixed in each nozzle toprovide the air mist applied to the strand 12.

[0047] Each water supply pipe 140 a, 140 b, 140 c, 140 d has anassociated respective control valve 146 a, 146 b, 146 c, 146 d, theadjustment of which changes the water flow rate and consequently the airmist flow rate for each spray group 137 a, 137 b, 137 c, 137 d. Eachsuch valve 146 a, etc. may be a butterfly valve or any suitableadjustable flow-rate valve. Each water supply pipe 140 a, 140 b, 140 c,140 d has an associated respective pressure regulator 155 a, 155 b, 155c, 155 d the adjustment of which regulates the water pressure throughthe associated supply pipes 140. Similar air control valves and airpressure regulators control flow rate and pressure for the air (notshown). The air and water control valves 146 and pressure regulators 155enable the spray characteristics of the sprays to be differentiallycontrolled transversely across the strand 12. Since the temperatureprofile of the strand is almost always symmetrical about its centerline,the choice of spray groups 137 a, etc. to include banks 130 equidistantfrom the center of the line is appropriate.

[0048] Preferably, each spray nozzle cluster 131, 124 comprises alongitudinally aligned series of individual nozzles 133 each being aninternal-mix pneumatic atomizing-type nozzle that mixes water and airfor discharging in a flat oval spray pattern. Each nozzle cluster 131,124 is preferably positioned so that the major axis of the oval spraypattern is transversely oriented, i.e. perpendicular to the line. Thetransverse width of each spray pattern and the distance between adjacentclusters 124 of nozzles are selected so that there is no gap butpreferably minimal overlap between the sprays of the adjacent clustersof nozzles. To this end, the nozzle clusters 124 in alternate columnsare offset from one another by a selected amount.

[0049] Because slabs or slab-shaped strands tend to cool naturally morequickly around the vicinity of their outer edges than at other parts ofthe surface, and because air mist sprayed on the longitudinal centralportions of the strand tend to migrate towards and contribute to furthercooling of the outer edges, transverse differential spray control of thecolumns or longitudinally aligned banks 130 enables a lower intensity ofspray to be applied by the outer banks of nozzles 130 than the innerbanks of nozzles 130. The spray characteristics of each spray group 137a, 137 b, 137 c, 137 d can be selected in response to this expectedtemperature profile and the heat-transfer properties of the associatedportion of the surface of the strand 12. Thus, by way of example, forquenching a given casting, spray group 137 a might be idle, spray group137 b providing a low flow rate spray, spray group 137 d providing aconsiderably higher flow rate spray, and spray group 137 c providing aspray at a flow rate intermediate that provided by spray groups 137 band 137 d. Suitable selection of flow rate and any other useful sprayparameters enables the temperature of all surface portions of the strand12 to be cooled to nearly the same post-quench temperature.

[0050] Masking means such as longitudinal flanges [not shown] can beoptionally installed on both longitudinal strand edges to shield theoutermost longitudinal edges of the strand from spray, thereby furtherreducing the amount of cooling effected on the strand edges. Thelongitudinal flange may be used in conjunction with the tranverselycontrollable sprays to reduce the amount of edge cooling.

[0051] Alternatively, suction means [not shown] such as longitudinalsuction slots extending along the length of the upstream quench station14 and at either longitudinal edge of the strand may be used to suctionexcess cooling fluid collected on the top surface of the strand, therebypreventing overcooling of the edge portions of the strand.

[0052] It has been found that it is unnecessary to provide spraysespecially to quench the sides of the strand 12 (for a strand to besevered into slabs); the side surfaces tend to cool sufficiently quicklythat separate spraying is unnecessary. Further, downstream edging maycorrect some surface defects in the vicinity of the side surfaces. Ifthere is a risk of overcooling the side edges of the steel, shields orspray masks in the vicinity of the side edges may be optionally providedto impede cooling fluid from reaching the side edges of the steel.

[0053] The air compressor 142, water pump 144 control valves 146 andpressure regulators 155 can be manually operated. An operator candetermine the appropriated spray characteristics required to apply asuitable quench from temperature profile data of the incoming slab 12,then manually make the appropriate adjustments for each of these piecesof equipment. Preferably, at least some of these steps are automated byconventional means. In this connection and referring to FIG. 4, monitorsor sensors for monitoring or measuring the values of selected parameterscan be provided. For example, basic supply water pressure and airpressure, line speed, pre-quench surface temperature of the steel acrossa transverse profile, post-quench surface temperature of the steelacross a transverse profile, and spray group flow rates or valvesettings could all be monitored or measured. The associated sensors areeach electrically connected to and communicative with a control unit160. For example, sensors 139, 141 for air and water supply respectivelytransmit data signals associated with air and water pressurerespectively to the control unit 160 via data transmission lines 143,145 respectively. The control unit 160 in response to the received datasignals can provide control signals via control signal lines 149, 151 toair pressure regulator 153 and water pressure regulator 155respectively, to remedy any irregularity in the air and water supplies.Suitable intervening digital/analog converters, relays, solenoids, etc.are not illustrated but would be used as required in accordance withconventional practice. The specific means chosen for the sensing ofsystem parameters and provision of data signals may be per seessentially conventional in character and is not per se part of thepresent invention.

[0054] Water control valves 146 and 147 control the water flow rate tobottom and top nozzle clusters 124, 131 respectively. Air control valves158, 159 control the air flow rate to bottom and top nozzle clusters124, 131 respectively. The air and water valves 146, 147, 158, 159 aresimilarly connected to and responsive to the control unit 160 whichcontrols the flow rate of air mist through the valves 146, 147 by meansof control signals transmitted via respective control signal lines, onlyone of which, line 157, is illustrated in FIG. 4 in the interest ofsimplification of the drawing.

[0055] Pyrometers 156 may be located downstream of the upstream quenchstation 14 or located in the vicinity of the quench unit exit port 127or elsewhere as the designer may prefer, e.g. just upstream of theupstream quench station 14. In FIG. 4, the strand 12 moves in thedirection of the arrow (right to left). The pyrometers 156 illustratedare mounted downstream of the upstream quench station above and belowthe as-quenched strand 12 passing therebetween. While only one block 156appears above and below the strand 12 in the drawing, it is to beunderstood that either the pyrometers 156 would be able to scan acrossthe transverse width of the strand 12, or else a transverse array ofpyrometers 56 across the width of the strand 12 would be provided. Foreach of the top and bottom strand surfaces, the pyrometers 156 measurethe transverse temperature profile of the respective surface. Thepyrometers 156 are electrically connected to and communicative with thecontrol unit 160 and transmit data signals associated with the surfacetemperature to the control unit 160 via data transmission lines 161following the strand's passage through the upstream quench station 14.With this data, the control unit 160 can determine whether theas-quenched temperature profile of the strand 12 falls within acceptableparameters; if not, the control program 160 (or the operator, ifperformed manually) calibrates the quench characteristics settingsaccordingly for the subsequent portions of the strand to be quenched.Generally, after enough data on castings of various compositions,widths, and casting histories have been accumulated, enough look-uptables for flow-rate settings will have been compiled that recalibrationwill seldom be necessary. Alternatively, pyrometers may be installedupstream of the upstream quench station 14 to determine the productsincoming temperature profile, thereby in conjunction with the downstreampyrometers 156 providing a dynamically responsive control system.

[0056] Roll speed tachometers 150 provide conveyor speed data to thecontrol unit 160 via data line 163. One or more tachometers 150 arepositioned at one or more selected conveyor rolls 22; in the case ofquenching of slabs, such tachometers 150 may be preferably located atboth upstream and downstream rolls 22 relative to the severing apparatus13 so that a measurement of both casting speed and strand conveyor speed(if permitted to be different from casting speed) is obtained. However,for purposes of simplification, only downstream tachometer 150 isillustrated in FIG. 4. The conveyer speed data are used by the controlunit 160 to determine the appropriate flow rate co be applied to thestrand 12, as described in further detail below.

[0057] Similarly, the tachometer 150 may with the control unit 160 bepart of a feedback control loop controlling the conveyor roll rotaryspeed. If line speed is to be made dependent upon quench operation, theconveyor roll drive (not shown) may receive control signals from thecontrol unit 160 that control the rotary speed of the conveyor rolls 22.For example, the control unit 160 may be programmed to change thecasting speed under certain circumstances, for example, if the castingspeed exceeds the quenching capacity of the upstream quench station 14;in this situation, the control unit 60 would send a control signal tothe caster 11 to reduce the speed of the caster 11.

[0058] In a preferred embodiment, the control unit 160 is a generalpurpose digital computer that is electrically connected to and receivesdata signals from sensed parameters, as exemplified by the various datasignal lines from the devices illustrated in FIG. 4. The control unit160 may have a memory storage device [not separately shown] for storingdata, and is operated by a suitable control program. Programming thecontrol program is routine and will take into account the specificobjectives to be served in any given rolling mill; such programming isnot considered to be per se part of this invention. For example, thecontrol program may conveniently be based in part on conventionaldynamic cooling control programs used in other parts of the castingmill, such as known cooling control programs used in the secondarycooling region of the strand containment and straightening apparatus 16.

[0059] Analysis indicates that preferred flow rate from a given nozzle,or bank or group of nozzles, is dependent upon casting speed roughly inaccordance with the equation

f=av ² +bv+c

[0060] where f is the flow rate for any given nozzle, or bank or groupof nozzles, a, b and c are constants, and v is casting speed. Obviouslythe constants a, b, C will be different for a given individual nozzle, agiven bank, or a given group. However, reliance should not be placed toohighly on the analytical results; empirical approaches are required todetermine optimum flow rate choices for nozzle groups.

[0061] Because the equation given above for the relationship betweenflow rate and casting speed includes one term that is proportional tothe square of the casting speed, it follows that dramatically increasingflow rates are required as casting speed increases. For example, theflow rate at a casting speed of 60 inches per minute for a 6-inchcasting might be roughly three times the flow rate required for the samecasting travelling at 30 inches per minute.

[0062] The control unit 160 may have user input devices such as akeyboard 162 to enable an operator to input new data or override any ofthe functions performed by the control program. For example, a test slabmay be occasionally removed from the casting line after the strand fromwhich it was cut was quenched and before it enters the reheat furnace.The cross-section of the test slab is then examined to determine (a)whether the steel's microstructure has been transformed by the quench toa suitable depth, and (b) whether the depth is suitably uniform acrossthe transverse width of the slab. If the operator is not satisfied withthe quench effected on the test slab, he may reprogram, adjust theweight to be given the parameters used by the quench program,recalibrate and recalculate look-up tables, or manually select the spraycharacteristics and any other controllable parameters, so thatsubsequent steel product is quenched to his satisfaction.

[0063] Referring to FIGS. 3 and 4, the transverse differential controlof the spray nozzles 124 enables the control unit 160 to tailor thetransverse width of the sprays to the width of the target strand 12 andto adjust flow rates of the spray groups 137 a, etc. to fit the surfacetemperature profile of the strand 12. The control unit 160 receives andprocesses a data signal identifying the width of the strand, determinesthe number of spray groups that are required to cover the targetsurfaces, and sends control signals to the appropriate output controldevices (e.g., solenoid valve actuators for the control valves) thatwill enable or disable the spray groups 137 a, etc. and adjust theirrespective flow rates.

[0064] The foregoing description has covered steady-state conditions inwhich the casting speed is constant. However, casting speeds typicallyvary considerably throughout a casting run. Since whenever the speedbegins to change, it is uncertain what new steady-state value of castingspeed will be reached, the flow-rate control system has to respond onthe basis of an inherent uncertainty as to the new target casting speedexpected to be reached after the current transient condition has come toan end. It has been found that potential deceleration-relatedover-quench problems tend to be more acute than potentialacceleration-related under-quench problems, partly because casting-lineproblems tend to require a fairly steep “ramp down” deceleration that issometimes as much as three times the rate of “ramp up” acceleration.Accordingly, the requisite decrease in flow rate to avoid over-quenchingshould be greater when deceleration occurs than the increase in flowrate when acceleration occurs in the casting line. In any givenfacility, an empirical approach should be taken to determine the optimumvalue. Monitoring surface temperature of the steel downstream of thequench may facilitate automatic or operator control of the flow ratethrough the quench nozzles. Typically the downstream surface temperatureshould be maintained in the range about 538° C. (1000° F.) to about 704C (1300° F.). At temperatures above about 1300° F., the quenched layertends to be insufficiently deep.

[0065] The arrangement offering the finest differential control over thespray characteristics of the sprays would include an array of nozzleshaving a dedicated supply line and control valve for each nozzle. Thisarrangement is within the scope of the invention but is not preferred,as the high number of individual controls may make the cost ofconstructing a upstream quench station prohibitive and the controlsystem for the array unduly complex.

[0066] The upstream quench station 14 may quench steel that includetitanium as an alloying element. In such cases, the relative position ofthe upstream quench station 14 in the line, its longitudinal dimensions,and the speed of the casting are preferably optimized to permitsubstantial TiN precipitation so that AlN precipitation is suppressedand solute nitrogen content is reduced. The presence of solute nitrogentends to reduce ductility in the cast metal. Typically, the steelcontains between about 0.015% and 0.040% titanium. Preferably, enoughtitanium is added to the metal prior to quenching to form atitanium-to-nitrogen weight ratio of the order of 3:1. Quenching to apost-quench surface temperature below about 750° C. to 800° C. yieldsoptimal TiN precipitation, thereby optimally suppressing ALN formation.As a further effect of optimal TiN precipitation, solute nitrogencontent is reduced. As a result, undesirable effects caused by AlNprecipitation are minimized. Other residual elements may precipitateand/or segregate to grain boundaries as the strand cools prior to beingquenched. Any contribution to surface defects by the other residualelements appears to be addressed either by the quench alone, or by somecombination of the quench and TiN precipitation. Also, the decrease inductility resulting from residual element precipitation is at leastpartially offset by the increase in ductility from the solute nitrogenreduction.

[0067] Referring back to FIG. 1, after the strand 12 has been quenchedin the upstream quench station 14, the strand 12 exits the upstreamquench station 14 and is severed into slabs 18 by the severing apparatus13. Then, each slab 18 is transferred onto a transfer table 20 thattransversely feeds each slab 18 sequentially into a reheat furnace 15,where the quenched portions of the slab 18 are reheated to a uniformtemperature at least above AC₃ (about 900 C for most steels of interest)and re-transformed to austenite. Preferably the slab is reheated to atemperature above T_(nr) and specifically, to a temperature of about1260° C. to provide a suitably high temperature for controlled rolling,discussed in detail below. It has been found that the austenite formedby this combination of quenching and reheating tends to have a finergrain size than austenite grains of a steel product that has not beenquenched before reheating. It has further been found that formation offiner grains of austenite is associated with the reduction in theoccurrence of defects in the surface of the eventual steel product. Theslabs 18 are held in the reheat furnace 15 for a period of timesufficient to heat the slabs is to a uniform temperature for rolling.

[0068] After the slab has been suitably quenched and reheated in thereheat furnace is, each slab 18 is transferred out of the reheat furnace15 and onto the upstream end of a rolling table 22. The slabs aredescaled in a descaler 17, which applies a series of high-pressure watersprays onto the surface of the slab to remove scale. If the weight of aslab exceeds the weight capacity of the coiler furnaces 21, 23, (or someother applicable limiting flow through parameter, to be discussed indetail below) the slab is severed by hot flying shear 25 into a targetportion within the coiler furnace weight capacity and a surplus portion.While a hot flying shear is the preferable choice, another severingdevice capable of severing such slabs may be suitable used. Preferablythe target portion is severed after it has been reduced to a thicknesswithin the coiler furnace thickness capacity, but if not, it is thenfurther reduced until its thickness is within the coiler furnacethickness capacity. Then the target portion is coiled in one of coilerfurnaces 21, 23 while the surplus portion can be further reduced by theSteckel mill, or immediately sent downstream for further processing.

[0069] Referring to FIG. 7, each slab is then sequentially toreversingly rolled in a Steckel mill 19 into an intermediate steelproduct 26 having a target end-product thickness (i.e. the objectiveend-thickness to be met) and a recrystallized and pancaked austeniticmicrostructure. This process is described in detail in U.S. Pat. No.5,810,951 and is summarized briefly here. In the Steckel mill 19 andduring a recrystallization stage, the slab 19 is first flat-passedrolled into an intermediate steel product at a temperature above T_(nr)in order reduce the thickness of the product by a selected amount and toenable some controlled austenite recrystallization in the product. Then,the product is subjected to at least one recrystallization coiler-passcomprising reducing the steel to a thickness within the coiler furnacethickness limitation (say, of the order of about 1″), and coiling anduncoiling the steel product within the coiler furnaces 21, 23. Thecoiler furnaces 21, 23 are maintained at least about 1,000 C, which isfor steel grades of interest, above the T_(nr). The coiler furnaces 21,23 substantially slow the natural (slow-air) cooling of the coiledproduct, so that the product remains above T_(nr) for the selectednumber of recrystallization coiler passes, thereby enabling additionalcontrolled austenite recrystallization of the product.

[0070] To provide enough time for recrystallization, therecrystallization stage should preferably be at least about 60 seconds;however, the desired recrystallization period will vary somewhat fordifferent steel chemistries. Typically, there is enough time for thesteel product to achieve the desired recrystallization period duringnormal flat and coiler passing; during the flat passes, the slower speedof the Steckel mill relative to conventional sequential in-line rollingstands affords an opportunity for recrystallization above T_(nr). Duringthe recrystallization coiler passes, the time taken to coil, stop,reverse direction, then uncoil the steel product provides additionaltime for recrystalliztion. The rolling sequence above the T_(nr) forthis period will achieve a fine-grained austenite structure of the steelundergoing sequential reductions.

[0071] Once the steel product 26 has been reduced to an interimthickness and sutficient recrystallization has occurred, the steelproduct 26 enters a pancaking stage wherein its temperature is permittedto drop below T_(nr) in a controlled manner during a further series ofcoiler passes through the Steckel mill, during which the fine grainstructure achieved is “pancaked” and consolidated. The coiler passesduring the pancaking stage are hereinafter referred to as pancakingcoiler passes to distinguish from the recrystallization coiler passes.Over the period of time taken by a predetermined series of pancakingcoiler passes, the temperature is permitted to drop from the T_(nr) tothe Ar₃ at which time the steel product 26 should have reached itstarget end-product thickness. Although a reduction of as much as 75%between the T_(nr) and the Ar₃ can be tolerated, it is preferred thatthe end-product thickness be about one-half the thickness of thefirst-rolled thickness of the intermediate steel product at the time itbegins to drop below the T_(nr). In other words, the “pancaking” rollingbetween the T_(nr) and the Ar₃ would preferably result in a 2.1reduction from the first-rolled thickness of the intermediate steelproduct to the end-product thickness.

[0072] In certain situations, it may be desired to slow down the rate ofnatural slow-air cooling of the steel product during the pancakingstage, e.g. if it will be difficult to achieve the desired reductionsduring this stage before the product falls below temperature Ar₃. Tothis end, auxiliary heaters may be installed at appropriate locationsaround the Steckel mill, such as an induction furnace [not shown] in aspace between the Steckel mill and pinch rolls of the coiler furnace.

[0073] In addition to facilitating the metallurgical treatment describedabove, the heat from the coiler furnace 21, 23 tends to equalize anytemperature variation that may have developed along the product'ssurfaces. The time taken to coil, reverse direction and uncoil theproduct typically provides enough time to equalize temperaturevariations found in steels typically processed in the Steckel mill 19.Should the product exhibit extraordinarily large temperature variations,the product may be held in the coiler furnace 21, 23 for a deliberatepause period to provide additional time for temperature equalization tooccur. Such temperature equalization is important for avoiding thedevelopment of inconsistent metallurgical properties after the productis quenched.

[0074] The pause periods, if carried out when the steel is above T_(nr)also provide additional time for desirable austenite recrystallization.In this connection, such austenite recrystallization pause periods maybe carried out during flat passing. However, the pause periods alsoprovide additional time for precipitates to come out of the steel, whichadversely affects the quality of the end-product. Therefore, an operatorwhen selecting the number and length of each pause period, if any, willtake into consideration these competing interests, and may affect theprecipitation problem somewhat by confining the occurrences of the pauseperiods to passes at higher temperatures during the recrystallizationperiod.

[0075] Preferred metallurgical practice dictates that the overallreduction in the rolling mill should be at least about 3:1. Accordingly,if the reduction imparted below the T_(nr) is about 2:1 (i.e. from theinterim-rolled thickness to the end-target thickness), then it followsthat the reduction above the T_(nr) should be at least about 1.5:1 (i.e.from a initial slab thickness to the interim-rolled thickness). Theamount of reduction, of course, will depend in large measure upon theratio of the end-product thickness (determined by the customer's order)and the initial slab thickness (typically fixed for a given rollingmill). If, for example, the end-product thickness is to be 0.5″, thenpreferably the intermediate steel product 26 is rolled from ainterim-rolled thickness of about 1.0″ to a thickness of 0.5″ below theT_(nr) to reach a rolling completion temperature of about the Ar₃. Ifthe initial slab thickness is 6″, it follows that a 6:1 reduction mustoccur above the T_(nr) in order to generate an intermediate product ofinterim-rolled thickness of 1.0″ that can be rolled between the T_(nr)and the Ar₃ to the desired 0.5″ end-product thickness.

[0076] Coiler furnaces based on present technology can typically coilsteel slabs having thicknesses up to 1.0″, although in some cases, steelproduct having thicknesses of up to 1¼″ may be coiled. Given that thedesired reduction from the interirm-rolled thickness to the end-productthickness is 2:1 (in the pancaking stage where the product temperatureis between T_(nr) and Ar₃), it follows then that the maximum end-productthickness that can be obtained is 0.5″. To obtain steel products with athicker end-product thickness, during the recrystallization stage, theproduct is subjected to flat pass rolling only, i.e. without anyrecrystallization coiler passes, at a reduction rate that achieves thedesired interim reduction while the steel product is above T_(nr). Forexample, if an end-product thickness of 0.75″ is desired, a 2:1reduction requires the interim-rolled thickness to be around 1.5″. Asthe product enters into the pancaking stage, i.e. falls below T_(nr),the product is further flat-passed until it reaches the targetend-product thickness. Should the end-product thickness be within thecoiler furnace thickness limitation,. it is possible to subject theproduct to at least one coiler pass; however, coiling product thickerthan 0.5″ is generally not desirable, as such product tends to sufferfrom coiling memory.

[0077] Steel product to be produced into flat plate may also undergorolling without coiler passes, especially at lower rolling temperatures,to avoid the risk of suffering coil memory.

[0078] To increase the rate of steel processing, an optionaloptimization method may be performed that involves processing a maximumweight slab through the rolling mill.

[0079] This maximum weight slab exceeds the capacity of one of therolling mill apparatuses but is within the maximum capacity of thereheat furnace 15. Typically for producing coiled plate, the limitingflow-through parameter is the weight capacity of the coiler furnace 21,23; typically for producing strip, it is the strip downcoiler 29. Forexample, if the weight of a maximum weight slab exceeds the weightcapacity of the coiler furnaces 21, 23, (or some other applicablelimiting flow through parameter, to be discussed in detail below) theslab is severed by hot flying shear 25 into a target portion within thecoiler furnace weight capacity and a surplus portion. Preferably thetarget portion is severed after it has been reduced to a coilablethickness, but if not, it is then further reduced until its thickness iswithin a coilable thickness. Then the target portion can be coiled inone of coiler furnaces 21, 23 while the surplus portion can be furtherreduced by the Steckel mill, or immediately sent downstream for furtherprocessing. This optimization method is discussed in detail in U.S. Pat.No. 5,706,688 and is briefly summarized below.

[0080] The weight of the maximum-weight slab to be rolled is typicallylimited by the maximum dimensions of the slab that can be reheated inthe reheat furnace 15, which can typically handle slabs of 6″ thickness,120″ width, and 75′ length. Such slabs of maximum dimensions weighapproximately 92 tons. While the Steckel mill 19 can be built to becapable of rolling such maximum weight slabs, the weight capacity of thecoiler furnaces 21, 23 is typically exceeded. Therefore, the maximumweight slab is severed into portions prior to coiling in the coilerfurnace 21, 23, wherein the weight of the portion to be coiled (thetarget portion) is within the coiler furnace weight capacity.

[0081] By positioning the flying shear 25 between the Steckel mill 19and the controlled cooling station 27, specifically, by positioning theflying shear 25 closely downstream of the downstream coiler furnace 23,a maximum-weight slab can be rolled by the Steckel mill 19 at atemperature around T_(nr), then be severed by the flying shear 25 intothe target portion and a surplus portion prior to coiling in the coilerfurnace 21, 23. Because the steel can be above T_(nr) when severed, ahot flying shear is preferred.

[0082] For producing plate, the maximum-weight slab is preferablyreduction rolled to below the coiler furnace thickness capacity beforeit is severed by the flying shear 25. The target portion is then coiledby one of the coiler furnaces 21, 23, and kept above T_(nr) inaccordance with the previously described steps of the method. Thesurplus portion is then further reversingly rolled in the Steckel mill19 to reduce its thickness to a desired end-product thickness, or istransferred immediately for downstream finishing.

[0083] The target portion is held in the coiler furnace 21, 23 for aselected period to enable substantial austenite recyrstallization andtemperature equalization, uncoiled, then is processed according to themethod described above.

[0084] In order for the surplus portion to he reversingly rolled in theabove manner, the target portion that is temporarily stored within oneof the coiler furnaces 21, 23 cannot protrude outside the mouth of thecoiler furnace 21, 23 to an extent that would cause interference withthe surplus portion during rolling. Referring to FIG. 5, the use of anauxiliary set of pinch rolls 241, 243 within the mouth each of thecoiler furnaces 21, 23, as proposed in the Smith U.S. Pat. No.5,637,249, facilitates the retraction of the intermediate product withinthe coiler furnace 21, 23 to an extent much greater than was previouslypossible using a conventional coiler furnace, and consequently the useof such auxiliary pinch rolls may be necessary or highly desirable inorder that the foregoing alternative mode of operation be practised toadvantage. Obviously, the foregoing procedure cannot be practised it thetongue of steel sheet hanging out of the coiler furnace mouth 235 is inthe path of travel of the residual portion of the steel beingflat-passed within the Steckel mill.

[0085] The objective of obtaining a final high-quality plate product bymeans of an economical sequence of steps in a mill provided with acost-effective selection of equipment is satisfied by the presentinvention. The plate flow-through capacity is typically determined bythe coiler furnace weight capacity. However, according to another aspectof the invention, at least part of the slab may be reduced to stripthickness for coiling on a downcoiler 29. This is illustrated in theleft column of the flowchart in FIG. 10. The slab is reduced to athickness not exceeding the coiler furnace thickness capacity, it issevered into a target portion of a weight not exceeding the stripdowncoiler weight capacity, and a surplus portion. The surplus portionmay be sent immediately downstream to be further processed as a flatplate product, or alternatively, the surplus portion may be furtherreduction rolled while the target portion yet to be rolled is held inthe coiler furnace (assuming the target portion thickness is less thanthe coiler furnace thickness capacity).

[0086] As a further alternative, one or both of the severed slabportions could be made into coiled plate product.

[0087] If desired, the benefit of processing a maximum weight slab maybe obtained independently of other advantages described in this method.For example, a maximum-weight slab may be severed into target andsurplus portions wherein the target portion is coiled in a downcoiler ascoiled plate and the surplus portion is sent directly downstream forfinishing. In this case, the surplus portion is not necessarilysubjected to controlled cooling, in order that the target portion befurther processed as quickly as possible. In such case, the surplusportion will not obtain optimal bainite microstructure. However, thebenefit of increased flow-through capacity is still achieved.

[0088] Referring back to FIG. 1, once the product 26 has been suitablyreduced in the Steckel mill, its leading end is cut off by hot flyingshear 25. The leading end is preferably cut into an precise vertical andclean transverse face. This facilitates even cooling of the top andbottom surfaces of the product when it is subjected to downstream forcedcooling (described in detail below). An unevenly cut face will result inone of the top and bottom surfaces being cooled before the other therebycausing uneven cooling between the two surfaces. If allowed to persist,such uneven cooling tends to cause the steel to buckle. The flying shear25 (itself of conventional design) has been found to be capable ofcutting a suitably precise and clean vertical transverse face so thatsuch buckling is avoided.

[0089] The flying shear 25 may also be used to cut the product 26 tolength (as separate from cutting the product to a target and surplusportion according to the optimization method). Once cut, the upstreamproduct portion is accelerated away from the downstream portion tocreate a suitable distance between the two portions. In some cases, suchspeed changes may cause longitudinal temperature variations along thesteel product when it is subjected to forced cooling in the temperaturereduction facility described in detail below. Such temperaturevariations if sufficiently severe tend to result in an inferior endproduct having inconsistent metallurgical and physical properties.

[0090] To avoid the onset of unacceptable longitudinal temperaturevariations, the location of the temperature reduction station can beextended further downstream to allow the product to reach a steady speedbefore being forcibly cooled; however, such lengthening is usuallyexpensive and impractical given the limited space in the mill and isinconsistent with the objective of maintaining a suitably hot productfor quenching. Alternatively, cutting to length may be effected by aseparate flying shear (“downstream flying shear”) located downstreamfrom the temperature reduction facility [not shown]. However, a secondflying shear will also be costly. Therefore, the product may be cut tolength by the upstream flying shear and a certain amount of temperaturevariation may be tolerated; in this connection the operator will bemindful to keep the acceleration of the leading portion to a minimum.

[0091] After the product is cut by the flying shear 26, the as-rolledsteel product enters a temperature reduction facility 27, 28 wherein itis forcibly cooled. The temperature reduction facility comprises aroller pressure quench unit 28 (“downstream quench station”) and acontrolled cooling station 27. The type of forced cooling effected willdepend on the type of steel selected for production. In this embodiment,two types are steel may be produced, each of which are subjected todifferent forced cooling: for martensite-rich steels, the product isquenched in the downstream quench station 28 and controlled coolingstation 27 then tempered in a tempering furnace; for bainite-richsteels, the downstream quench station 28 is de-activated and the productis cooled in the controlled cooling station 27 only.

[0092] To produce quenched and tempered steels, the product is fed intothe downstream quench station 28 for quenching. The preferred minimumquench start temperature of the product is Ar₃; as discussed above, therolling schedule is selected so that the as-rolled product is a suitablyhigh temperature for quenching. Referring to FIGS. 8 and 9, thedownstream quench station 28 is a roller pressure type quench (RPQ) unitthat within its housing 60 includes a plurality of tightly-spacedrollers 62 arranged above and below the product passing in-between. Therollers 62 feed the product through the downstream quench station 28 andapply a constraining force above and below the product. There is insidethe downstream quench station 28 near the quench apparatus entrance end,opposed upper and lower headers 64, 66 arranged above and below theproduct passing in-between. The headers 64, 66 are aligned transverselyto the rolling direction and emit a transverse uniform sheet of highvelocity cooling water onto the upper and lower surfaces of theintermediate steel product, respectively. To facilitate the emission ofa uniform sheet of water, the headers 64, 66 may comprise of a pluralityof discrete emitters [not shown].

[0093] The sheet of water emitted by each header 64, 66 is deflected bya respective deflector 68, 70 at an angle that prevents water fromspitting upstream of the plate thereby causing pre-cooling. It has beenfound that an angle of about 22 degrees to the vertical in the directionof travel of the product provides suitable deflection. The tips of theheaders 64, 66 are about {fraction (5/8)}″ from the top and bottomsurfaces of the product passing in-between. The headers 64, 66 arecarefully aligned so that the leading upper and lower edge of the steelare simultaneously struck by the water emitted from the headers 64, 66.This provides even and uniform cooling to the upper and lower surfacesand in conjunction with the constraining force provided by the rollers62, reduces the risk of the product buckling.

[0094] Additional quench water is delivered to the upper and lowersurfaces by a series of upper and lower downstream headers 72, 74.

[0095] The features of the downstream quench station discussed above areconventional and available in commercial quench units, such as theroller pressure continuous high intensity quench units designed andmanufactured by Drever Company (“Drever RPQ unit”).

[0096] The accumulation of water on the top product surface tends tocause non-uniform quenching of the top product surface relative to thebottom product surface. Severe non-uniform quenching may cause the steelto buckle or produce inconsistent as-quenched properties between the topand bottom surfaces. To avoid this, a suction device 76 may be installedimmediately downstream of headers 72, 74 or elsewhere above the topproduct surface where space permits. The suction device has atransversely-aligned slot spanning the width of the maximum widthproduct and uniformly suctions water off the product passing underneath.To facilitate uniform suction, the slot may comprise of a plurality ofsub-slats closely spaced in a transverse row.

[0097] Each of rollers G2 may be separately driven by associated rollerdrives [not shown], similar to the arrangement in conventional hotlevellers. This provides independent speed control to each individualroller 62 (or at least two separate sets of rollers driven by twoseparate roller drives). The rollers 62 may be operated at slightlydifferent speeds to create a lengthwise tension to the product 26passing through. Such tension has been found to contribute to improvedproduct flatness, and possibly, to improved surface properties.

[0098] In operation, cooling water is discharged through the headers 64,66 at a rate and pressure sufficient to reduce the temperature of therespective surfaces of the steel product below the martensitetransformation start temperature M_(s) and to effect a surface coolingrate exceeding the martensite critical cooling rate, therebytransforming the upper and lower surface layers of the steel product tomartensite. The depth of transformation will vary from product toproduct depending on a number of factors as including, the thickness ofthe product, the cooling rate, pressure, and cooling fluid temperature.The Drever RPQ unit is operable to deliver a quench at 100 psi and 3500gal/mi of cooling water. This quench has been found to generate asurface layer of martensite of around 0.25″ deep.

[0099] After exiting the downstream quench station 28, the product'supper and lower surface layers have been cooled to about the temperatureof the cooling fluid; however the product core remains relatively hot,typically at or exceeding Ar₃. The product 26 is then fed through thecontrolled cooling station 27, wherein it is subjected to furthercooling directed at keeping the product surface at a chilledtemperature. This creates and maintains a maximum temperature gradientthat enables maximum heat dissipation out of the product core, as wellas impedes the tendency for heat from the core to temper the surfacelayers. By so cooling the steel at its maximum heat transfer rate, ahigh proportion of martensite is obtained in the end product.

[0100] The controlled cooling station 27 includes an upper array 51 oflaminar flow cooling devices that provide cooling water to the uppersurface of the intermediate steel product 61 passing underneath theupper array 51. At the same time, a lower array 53 of spray coolingdevices provide a cooling spray to the undersurface of the intermediatesteel product 61 passing above the array 53. The upper array 51comprises a longitudinally arranged series of cooling nozzle groups orbanks 55. The lower array 53 comprises a series of spray headers 57. Theheaders 57 are themselves longitudinally spaced from one another andinterposed between a longitudinal series of transversely extending tablerolls [not shown] that support and drive the product. A suitable suchcontrolled cooling station 27 is discussed in detail in U.S. Pat. No.5,810,951.

[0101] Preferably, for all portions of the product, the heat dissipationrate exceeds the critical martensite quench rate and the finishtemperature is below the martensite start temperature, so that theentire product is transformed into martensite. However, since the amountand rate of heat dissipation depends on many factors, including theinherent heat transfer characteristics of the steel, and the steel'schemistry and dimensions, not every portion of the productmicrostructure may necessarily be transformed into martensite. Anoperator will consider these factors in relation to the capability ofthe downstream quench station 26 and controlled cooling station 27 toensure that the end-product will be sufficiently transformed so that itfalls within the applicable quench and temper specifications for thesteel end-product. Such specifications may be, for example, ASTM 514 orASTM 514M.

[0102] After cooling in the controlled cooling station, the as-quenchedproduct should have been cooled to about the cooling fluid temperature(typically about 90 F) on the surface and 200 F in the core.

[0103] Referring back to FIG. 1, after leaving the controlled coolingstation 27, the product is either downcoiled on a downcoiler 29 if thesteel is to be eventually processed into strip end product, or passedfurther downstream for further processing as an eventual plate product.The remaining discussion relates to the production of plate product.

[0104] The product may be optionally levelled in a hot leveller 31.Then, the product is transferred to a transfer table 33 and thencetransversely to a cooling bed 35. Then, the steel is taken off-line andtransferred into a tempering furnace 37 where it is held for a suitabletempering period at a suitable tempering temperature. Tempering effectsa useful amount of ductility to the product, without which theas-quenched product would be undesirably brittle. The temperingtemperature is selected to be below a lower critical transformationtemperature Ar₁ being the lower temperature limit above which austenitetransforms into austenite. The tempering effects a controlled diffusionof entrapped carbon from the martensite to restore some ductility to theproduct. Suitable such tempering furnaces are commercially available andnot per se part of the invention.

[0105] After tempering, the product has extremely high strength andother properties characteristic of quench and tempered steels. Theproduct is then transferred to a plate finishing line, which typicallyincludes a static shear 39 for cutting heavier plate product to length,a cold-leveller station 41 for further levelling, and a flying shear 43for cutting or trimming lighter plate product to length. Afterfinishing, the finished plate product may be piled in piles 49, or putonto transfer tables 47 for shipping.

[0106] For processing non-quench and tempered steel, the forced coolingdownstream of the Steckel mill is directed towards obtaining a productwith a predominantly bainitic microstructure; such a product tends toexhibit enhanced strength and toughness. To produce such product, theroller-pressure downstream quench station 2B is deactivated and theas-rolled product 26 is fed through the downstream quench station 28without quenching and into an operational controlled cooling station 27.The product 26 will have slow air cooled somewhat by the time the itreaches the controlled cooling station 27, but it is still relativelyhot, in the order of about the Ar₃, and therefore has a predominatelyaustenitic microstructure. In the controlled cooling station 27, theproduct surface is cooled at a rate of about 12 C to about 20 C persecond and to a temperature of about 200 C to about 350 C below Ar₃.This cooling transforms most of the austenite into fine-grained bainiteso that the product has a predominantly bainitic microstructurerelatively free of martensite, which in conjunction with the austeniterecrystallization and pancaking effected during rolling, provides thesteel with enhanced strength and toughness. This method is described indetail in patent and U.S. Pat. No. 5,810,951.

[0107] After cooling in the controlled cooling station 27, the productis further processed and finished in a manner substantially similar tothe processing and finishing of quench and tempered steel describedabove.

[0108] An exemplary application of the invention to process {fraction(1/2)}″ 80,000 PSI yield-strength steel plate begins with a 6″ slab ofthe following chemistry: carbon 0.03 to 0.05% manganese 1.40 to 1.60%sulphur 0.005% max phosphorus 0.015% max silicon 0.20 to 0.25% copper0.45% max chromium 0.12% max columbium (niobium) 0.02 to 0.06%molybdenum 0.18 to 0.22% tin 0.03% aluminum 0.02 to 0.04% titanium 0.018to 0.020% nitrogen 0.010% max vanadium up to 0.08%

[0109] Consider the above 6 inch thick steel casting having a variablewidth of anywhere between about 40 inches and 125 inches, being producedat normal casting line speeds of anywhere between about 30 inches perminute and 75 inches per minute. Assume that a quench penetration of atleast about a half-inch from the surface is targeted, and that thequench will reduce surface temperature of the casting from a temperatureof the order of 982 C (1800° F.) to a temperature of the order of538-704 C (1000-1300° F).

[0110] Engineering considerations, notably the principle ofsimplification, make it desirable to control nozzles in banks oflongitudinally aligned nozzles. Four groups of top nozzle banks can bearrayed over the maximum width of the casting, including:

[0111] first, a central group of at least 1, and perhaps 3 or 5 banks ofnozzles;

[0112] second, a mid-inner group comprising, say, 4 banks of nozzles,two on either side of the centre line and lying outside the centralgroup;

[0113] third, a mid-outer group of nozzles comprising, say, 4 nozzlebanks, two on either side of the centre line and outside the mid-innergroup; and

[0114] fourth, a final outermost group of nozzles comprising, say, 4banks, two on either side of the centre line, and the outermost bank ofwhich on each side of the centre line overlaps the edge margin of thecasting of maximum width, or may be inset slightly from the edge of thecasting.

[0115] A counterpart four groups of bottom nozzle banks can be arrayedunder the casting in a comparable manner. Note that the maximum numberof nozzle banks in the foregoing example exceeds the number illustratedin FIG. 3.

[0116] With a nozzle array and nozzle bank selection of the foregoingsort, it may be useful to operate all four groups of top and bottomnozzles only when the casting being produced is of maximum width, or upto about, say, 90% of maximum width. For castings of, say, 75-90% ofmaximum width, the outermost group of nozzles would be idled. Forcastings of about 55-75% of maximum width, the outermost group and themid-outer group of nozzles could be idled. For castings of about 35-55%of maximum width, all nozzle groups except the central group could beidled.

[0117] Conveniently, the bottom nozzles underneath the casting maycorrespond on a one-to-one basis with the top nozzles above the casting.The groups of bottom nozzles can operate at flow rates that mayconveniently be set at a specified multiple of the flow rates of thecorresponding groups of top nozzles. It has been found that the flowrate for the bottom nozzles should be preferably from about 1.2 to about1.5 times the flow rate for the top nozzles located above the casting.The reason for the difference, of course, is that water or other coolingfluid is assisted by gravity to cool the top of the casting, but waterquickly falls away from the bottom surface of the casting.

[0118] It may be desired to set the flow rates for the different groupsof nozzles at specified fractions of the central group. The fractionchosen will depend upon how many groups there are altogether, andwhether particular groups are operating, or idle. It has been foundeffective to have the outermost nozzle groups provide flow rates thatcan be as little as about {fraction (1/4)} the flow rate of the centralnozzle group, with the fractions for nozzle groups between the outermostgroup and the central group progressively increasing in relative flowrate as one progresses from the transverse edge of the nozzle arraytoward the central nozzle group (which coincides with the centralportion of the casting being sprayed). For example, the mid-inner nozzlegroup next to the central group might be operated at about 50 to 75% ofthe flow rate of the central group of nozzles. Different ratios may bechosen for the top and bottom arrays of nozzles respectively, butgenerally similar ratios have in practice proven to be satisfactory fora given top nozzle group and its counterpart underneath the casting,relative to the central nozzle group in the two cases.

[0119] It has also been found that if nozzle groups are selected asindicated above, and idled selectively as indicated above, it may bepossible to have all three nozzle groups other than the central nozzlegroup operate at a single specified fraction of the flow rate of thecentral nozzle group, the fraction preferably being in the range about50-75% of the flow rate provided by the central nozzle group. Transversecontrol of flow rate in this mode of operation is effected byselectively idling one or more groups of nozzles.

[0120] Values chosen for flow rates, selection of nozzle groups toremain idle, and other operating parameters may be expected to varydepending upon steel grade. For most commercial grades of steel platecast from a 6″ mold, a quench penetration into the casting of about{fraction (1/2)}″ is a satisfactory. The total flow required will varyconsiderably with casting width; for narrower castings of up to about65″, it may be possible to achieve quite satisfactory quenching withonly the central nozzle groups (top and bottom) operating. Formaximum-width castings of, say, 125″, all nozzle groups should operatefor at least moderate casting line speeds (say 30″/min and over). At acasting line speed of 30″/min, the top central nozzle group of threelongitudinal banks of nozzles might provide a flow rate of about 120gal/min; at 60″/min, that same group might provide a flow rate of aboutthree times the flow rate set for 30″/min. The actual choices of settingof flow rate per nozzle group are best determined empirically for eachspeed, for each casting width, and for each grade of steel beingproduced. A set of look-up tables may be compiled based on the empiricaldata and used as input to the computer for controlling nozzle groups orused by the mill operator to set flow rates, or in unusual orexperimental circumstances to override the computer where this isconsidered desirable. Computer control of solenoids or relays or thelike for controlling butterfly valves or other suitable valves forindividual nozzles or groups of nozzles is known per se and not per separt of the present invention. If desired, appropriate instrumentation,such as pyrometers, may be located at the quench unit 14 entrance andused to construct a temperature profile model of the incoming steelproduct. This model would be updatable with fresh date from theinstrumentation and would be utilized by the control unit 160 todynamically control the operation of the quench.

[0121] For automatic control of the quench, the quench control programmay be alternatively developed from known cooling control models, suchas those developed by Richard A. Hardin and Christoph Beckermann fromthe University of Iowa, or I. V. Samarasekera et al. tram the Universityof British Columbia. The programming of the control program from suchknown control models or known cooling: control programs is routine.

[0122] After quenching, the slab is sent to a reheat furnace wherein itis heated to a uniform rolling temperature of preferably above or about1,260 C.

[0123] The slab is then sent to the Steckel mill for reverse rollingaccording to the following rolling schedule: Temperature Thickness SlabDropout 1,260 C. 6.0″ (252.4 mm) 1,230 C. 4.7″ (119.4 mm) 1,200 C. 3.5″(88.9 mm) 1,165 C. 2.4″ (61.0 mm) 1,100 C. 5 1.6″ (40.6 mm) 1,050 C.1.0″ (25.4 mm) T_(nr) (Non-Recrys.)   970 C. COIL in Coiler Furnace  950 C. 0.76″ (19.0 mm)   875 C. 0.61″ (15.5 mm)   800 C. 0.50″ (12.7mm) Ar₃ (Upper   800 C. 0.50″ (12.7 mm) Critical)

[0124] In the above table, for steel of the chemistry indicated, theT_(nr) is approximately 970 C. During the recrystallization stage, thesteel product is reduced by a series of flat passes according to theabove rolling schedule from the reheat furnace dropout temperature of1,260 C to 1,050 C. After the flat passes, the steel product in a singlerecrystallization coiler pass is reduced to the interim thickness of1.0″ and coiled in one of the coiler furnaces. Both coiler furnaces aremaintained at an interior furnace temperature of 1,000 C (but at least970 C) to prevent the steel being rolled from dropping in temperaturebelow the T_(nr) before being reduced to the selected interim thickness.

[0125] The steel product preferably stays in the coiler furnace for aperiod that in combination with the flat passes totals at least 60seconds. While the above rolling schedule has only one recrystallizationcoiler pass, it is also acceptable to have multiple recrystallizationcoiler passes, so long as the total time spent above T_(nr) is at least60 seconds (or such other period as is suitable to the chemistry of thesteel being rolled). However, it is preferable to have only one coilerpass, as this permits the Steckel mill to process another slab (surplusportion) while the first slab (target portion) is held out of the way inthe coiler furnace.

[0126] Once the intermediate steel product has fallen toT_(nr), itenters the pancaking stage where it is rolled in a series of pancakingcoiler passes between T_(nr) and the Ar₃, (800 C in the above example).During the pancaking stage, the first-rolled thickness of 1.01″ at aboutthe T_(nr) (which should still be effective for achieving some degree ofrecrystallization,) is successively reduced. Note that rolling below theT_(nr) will not admit of any further recrystallization, but instead thenext rolling sequence pancakes or flattens the crystal structurepreviously obtained. In this example, the initial 1.0″ thicknessobtained from rolling at the T_(nr) is reduced by 50% to an end-productthickness 0.50″ at the Ar₃. This 2:1 reduction in thickness from theT_(nr) thickness to the Ar₃ thickness is representative, and tends togenerate a preferred degree of pancaking of the fine crystal structurechat had been obtained in the austenite (that is, in accordance with theprocedure described, transformed predominantly into bainite).

[0127] In the above discussion, the assumption has been made that theT_(nr) and the Ar₃ can be accurately determined for a given steelproduct. However, different and somewhat competing approaches to thedetermination of these critical temperatures are discussed in thetechnical literature. Depending upon the equations used, the calculatedAr₃ (for example) computed according to a given method may differ by asmuch as about 10 C from the calculation of the Ar₃ using one of thecompeting methods of calculation. The present invention is notpredicated upon any particular selection of method of calculation of theT_(nr) or Ar₃. A 10 C variation at either end of a stated range oftemperatures is equally considered not to be material to the practice ofthe present invention. In any given plant, the metallurgist or theperson responsible for mill operation will undoubtedly evaluate rollingand cooling results empirically, and choose a combination of rolling andcooling parameters that appears to give optimum or near-optimum results.However, optimum or near-optimum results should be obtainable with aminimum of empirical adjustment using the combination and methodsdescribed and claimed in the present application.

[0128] After rolling, the product is subjected to forced coolingsuitable for the type of end-product steel desired, e.g.quench-and-tempered steel or bainite-rich steel.

[0129] Alternative Embodiments

[0130]FIGS. 5 and 6 illustrate an alternative embodiment of the upstreamquench station 14 that includes longitudinal spray control. In thisembodiment, there is a second top and bottom arrays of nozzle clusters170, 172 interspersed with the top and bottom nozzle arrays 126, 128 ofthe first embodiment, i.e. the array of nozzles that are actuated on atransversely variable basis. For purposes of distinction, the second topand bottom arrays are hereinafter referred to as thelongitudinal-control arrays, and the arrays ot the first embodimentillustrated in FIGS. 1-4 are referred to as the transverse-controlarrays.

[0131] The longitudinal-control arrays are actuated on a longitudinallyvariable basis. To this end, there are opposed top and bottomlongitudinal-control arrays of nozzles 170, 172 (FIG. 5) above and belowthe strand 12, respectively. For convenience, the bottomlongitudinal-control array 172 is discussed, it being understood thatthe discussion also applies to the top longitudinal-control array 170.The longitudinal-control array 172 comprises a plurality of separatelongitudinally-spaced banks 176 a, 176 b, 176 c of transversely alignednozzles (“longitudinal nozzle banks”) each having dedicated supply pipes182 a, 182 b, 182 c that are arranged in a horizontal plane below thebottom transverse-control array 128. Each nozzle 178 of eachlongitudinal nozzle bank extends from its respective supply pipe 182 aetc. into the same plane as the nozzles 133 from the bottom transversecontrol array 128. Each longitudinal nozzle bank 176 spans a width thatis at least as wide as the maximum strand width. The nozzles 178 providespray patterns complementary to the spray patterns provided by thetransverse-control nozzle array 128. The arrangement illustrated isexemplary; more longitudinal-control nozzle banks could be provided;more nozzles altogether of smaller capacity and providing smaller spraypatterns could be provided, etc.

[0132] In this embodiment, the longitudinal supply pipes 182 areconnected to associated respective water control valves 184 a, 184 b,184 c and water pressure regulators 185 a, 185 b, 185 c. Similarly, thelongitudinal supply pipes are connected to associated respective aircontrol valves and pressure regulators (not shown) In a manner similarto the transverse spray control described in the first embodiment, thecontrol valves 184 and pressure regulators 185 regulate the fluid flowrate and pressure for the three longitudinally spaced banks 176. Suchlongitudinal control is useful in countering non-uniform longitudinalcooling in the strand, which may for example, be caused by anomalies inthe orderly progress of the steel through the caster assembly 21. Forexample, for a given length of the strand, the leading portion may be ata higher temperature than the trailing portion at a given line location.In this connection, the longitudinal-control array may be programmed toapply a higher intensity quench to the leading portion of the strand,and a lower intensity quench to the trailing portion. As the lengthwisestrand portions are moving through the upstream quench station 14, thequench intensity for each longitudinally spaced group must be varieddepending on which strand portion is directly above (or below for thetop longitudinal array 170).

[0133] Optionally, the flow rate provided by each longitudinal arraynozzle 178 near the center line of the strand may be somewhat largerthan that of nozzles 178 near the strand edges. Suitable sizing of thenozzles 178 in the banks 176 can achieve this objective. This variationin flow rate across the bank enables a higher coolant flow rate to beprovided by the central nozzles 178 than the outermost nozzles 178,thereby providing a differential transverse cooling to complement thevariable control transverse cooling described in the first embodiment,albeit without fine transverse control of the longitudinal-controlnozzles. The chosen transverse flow-rate profile would be selected tomatch within engineering limits the transverse surface temperatureprofile of an average casting.

[0134] The upstream quench station 14 in accordance with this embodimentmay be alternatively located downstream of the severing apparatus 13.The steel product that enters the upstream quench station 14 will insuch case typically be in the form of slabs severed by the severingapparatus 13. The data and control program parameters of the controlunit are appropriately modified to account for the longer distancebetween the caster assembly 21 exit and the upstream quench stationentrance 123, and the time it takes the strand to travel this distance.Locating the upstream quench station 14 further downstream from thecaster assembly 21 enables the steel product to cool somewhat in ambientair before it reaches the upstream quench station 14, thereby reducingthe amount of water and energy required to quench the product surfacesto the appropriate temperature.

[0135] If the upstream quench station 14 is located downstream of thesevering apparatus 13, the casting line speed should preferably be keptconstant between the caster assembly 21 and reheat furnace 15. As thesteel product has been severed into slabs, the casting line speed of theslabs can be changed relative to the casting line speed for the strand.However, when such a speed change occurs, slabs tend to develop alongitudinal temperature gradient. For example, if the speed of thecasting line downstream of the severing apparatus increases, the steelproduct that has exited the caster assembly 21 but not yet entered theupstream quench station 14 will have a downstream portion that will havehad more time to cool than an upstream portion. In a typical continuouscasting mill, the casting line speed remains fairly constant between thecaster assembly 21 and the reheat furnace 15, and therefore, theoccurrence of such longitudinal temperature gradients is minimal.However, should there be a longitudinal temperature gradient, suchgradient can be minimized or eliminated by use of the longitudinal spraycontrol described above.

[0136] In a further alternative embodiment, a portion of the upstreamquench station 14 is installed within the strand containment andstraightening apparatus 16 near the caster assembly exit, and operatesin conjunction with a portion of the upstream quench station 14positioned outside the caster assembly 21 to quench the steel product ina manner described for the above two embodiments. Of course, the strand19 must be completely unbent and straightened before it is quenched.

[0137] The location of the upstream quench station 14 in this embodimentis selected to be closely downstream of the caster assembly 21 tominimize the formation of lengthwise and transverse temperaturevariations along the surfaces of the strand. Such temperature variationsif not compensated for tend to cause inconsistent as-quenched propertiesin the steel. Should temperature variations along the product reach anunacceptable severity, the upstream quench station is fitted with acontrol system and equipment that provide a controlled spraying systemthat compensates for the temperature variations, so that after quenchspraying in the upstream quench station, the sprayed surfaces have auniform temperature and the surface layers have a microstructure that istransformed to a uniform depth.

[0138] Other alternatives and variants of the above described methodsand apparatus suitable for practising the methods will occur to thoseskilled in the technology.

1. In or for use in an in-line rolling mill for producing steel, saidmill having a continuous caster for producing a cast strand of steel,severing means for cutting the cast strand transversely into a series ofslabs, a reheat furnace downstream of the caster for bringing slabs to asubstantially uniform pre-rolling temperature, and a Steckel milldownstream of the reheat furnace for rolling the castings in sequence;the apparatus combination comprising; (a) an in-line upstream quenchstation located downstream of the caster and upstream of the reheatfurnace and having a plurality of spray nozzles directed at the caststeel for applying cooling sprays onto the cast steel that quench asurface layer of the cast steel to a selected depth so that the surfacelayer is transformed from an austentitic to a substantiallynon-austentitic microstructure; (b) a shear located downstream of theSteckel mill for transversely severing and trimming the leading edge ofthe rolled steel to provide a precise transverse vertical face thereonand for optionally cutting the steel to a series of portions of selectedlength; and (C) a temperature reduction station downstream of the shearfor applying a controlled flow of cooling fluid to the rolled steel soas to obtain a preferred microstructure of the steel; wherein (d) thereheat furnace heats the slabs to a suitable pre-rolling temperatureabove the temperature T_(nr), and transforms the quenched surface layerto fine-grained austenite; and (e) the Steckel mill rolls and reducesthe thickness of the slab first in a temperature range above thetemperature T_(nr) and then at a decreasing temperature between thetemperatures T_(nr) and Ar₃ to obtain first a controlledrecrystailization of austenite and then a pancaking of the austenite. 2.The apparatus of claim 1, wherein the temperature reduction stationcomprises a controlled cooling station for cooling the rolled steel at arate of about 12 C to 20 C per second and to a temperature of about 200C to about 350 C below the temperature Ar₃, thereby obtaining in therolled steel a preferred microstructure including a substantial portionof fine-grained bainite.
 3. The apparatus of claim 1, wherein thetemperature s reduction station comprises a downstream quench stationimmediately followed by a controlled cooling station, the quench stationapplying cooling fluid to the rolled steel at a rate and in a quantitysufficient to quench the rolled steel rapidly and intensely to obtain apreferred microstructure including a substantial portion of fine-grainedmartensite, and the controlled cooling station applying additionalcooling fluid sufficient to maintain the cooling of the steel at a highrate so as to obtain a relatively high portion of fine-grainedmartensite in the quenched steel.
 4. The apparatus of claim 3,additionally including a tempering furnace receiving the rolled steelfrom the controlled cooling station and tempering same.
 5. In or for usein an in-line rolling mill for producing steel plate of a selectedtarget length and thickness, said mill having a continuous caster forproducing a cast strand of steel, severing means for cutting the caststrand transversely into a series of slabs, a reheat furnace downstreamof the severance means for bringing slabs to a substantially uniformpre-rolling temperature, and a Steckel rolling mill downstream of thereheat furnace for rolling the castings in sequence; the apparatuscombination comprising: (a) an in-line upstream quench station locateddownstream of the caster and upstream of the reheat furnace and having aplurality of spray nozzles directed at the cast steel for applyingcooling sprays onto the cast steel that quench a surface layer of thecast steel to a selected depth so that the surface layer is transformedfrom an austentitic to a substantially non-austentitic microstructure;and (b) a temperature reduction station downstream of the Steckel millfor applying a controlled flow of cooling fluid to the rolled steel soas to obtain a preferred microstructure of the steel; wherein (c) thereheat furnace heats the castings to a suitable pre-rolling temperatureabove the temperature T_(nr) and transforms the quenched surface layerto fine-grained austenite; (d) the Steckel mill rolls and reduces thethickness of the casting first in a temperature range above thetemperature T_(nr) and then at a decreasing temperature between thetemperatures T_(nr) and Ar₃ to obtain first a controlledrecrystallization of austenite and then a pancaking of the austenite. 6.The apparatus of claim 5, including a transverse shear in line with andproximate to the temperature reduction station.
 7. The apparatus ofclaim 5, including a transverse shear upstream of the temperaturereduction station for transversely severing and trimming the leadingedge of the rolled steel to provide a precise transverse vertical facethereon.
 8. The apparatus of claim 6, additionally including atransverse shear downstream of the temperature reduction station forcutting the steel to a series of portions of selected length.
 9. Theapparatus of any of claims 5-8, wherein the temperature reductionstation comprises a controlled cooling station for cooling the rolledsteel at a rate of about 12 C to 20 C per second and to a temperature ofabout 200 C to about 350 C below the temperature Ar₃, thereby obtainingin the rolled steel a preferred microstructure including a substantialportion of fine-grained bainite.
 10. The apparatus of any of claims 5-8,wherein the temperature reduction station comprises a downstream quenchstation immediately followed by a controlled cooling station, the quenchstation applying cooling fluid to the rolled steel at a rate and in aquantity sufficient to quench the rolled steel rapidly and intensely toobtain a preferred microstructure including a substantial portion offine-grained martensite, and the controlled cooling station applyingadditional cooling fluid sufficient to maintain the cooling of the steelat a high rate sufficient to substantially maintain and preferably toincrease the portion of fine-grained martensite obtained in the rolledsteel.
 11. The apparatus of claim 10, additionally including a temperingfurnace receiving the rolled steel from the controlled cooling stationand tempering same.
 12. The apparatus of any of the preceding claims,wherein the Steckel mill is provided with coiler furnaces immediatelyupstream and downstream thereof, each said coiler furnace includingpinch rolls in the vicinity of the entrance port thereof forfacilitating near-complete retraction into the coiler furnace ofcoilable steel undergoing rolling.
 13. The apparatus of any of thepreceding claims, wherein the controlled cooling station provideslaminar flow cooling for the upper surface of the rolled steel andquasi-laminar flow cooling for the undersurface of the rolled steel. 14.The apparatus of any of the preceding claims, wherein said upstreamquench station comprises: (a) an array of spray nozzles arranged intransversely separated spray groups above and below the cast steel asthe cast steel passes through the upstream quench station; (b) at leastone valve for each spray group for controlling the amount of sprayprovided by each group to the cast steel; (c) a control unit forcontrolling the valves thereby to regulate the amount of spray providedby each spray group, in response to selected parameters includingcasting width and casting speed; thereby to provide transverselydifferential spray to the cast steel being quenched.
 15. The apparatusof claim 14, wherein the selected parameters include a post-quenchsurface temperature profile of the cast steel.
 16. The apparatus ofclaim 14 or 15, wherein the selected parameters include a pre-quenchsurface temperature profile of the cast steel.
 17. The apparatus of anyof claims 14-16, wherein the upstream quench station is located upstreamof the severing means.
 18. The apparatus of any of claims 14-16, whereinthe upstream quench station is located downstream of the severing means.19. The apparatus of any of claims 14-18, wherein the array of nozzlesunderneath the cast steel is substantially the mirror image of the arrayof nozzles above the cast steel.
 20. The apparatus of any of claims14-19, wherein the array of nozzles underneath the cast steel provides agreater amount of spray to the cast steel than is provided by thenozzles above the cast steel.
 21. The apparatus of any of claims 14-20,additionally including spray nozzles arrayed in longitudinally spacedtransversely extending groups, said last mentioned groups beingcontrolled by the control unit to provide longitudinally differentialspraying of the cast steel.
 22. An in-line method for producing a rolledsteel product, including continuously casting a strand of steel,severing the cast strand transversely into a series of slabs, reheatingthe slabs to a substantially uniform pre-rolling temperature, andreversingly reduction-rolling the reheated steel slabs; characterizedby: (a) applying to the cast steel an upstream quench prior to reheatingso as to quench a surface layer of the cast steel to a selected depth sothat the surface layer is transformed from an austentitic to asubstantially non-austentitic microstructure; (b) shearing the leadingedge of the rolled steel immediately after completion of rolling to cropthe steel so as to provide a precise transverse vertical face on theleading edge of the rolled steel; and (c) applying to the cropped rolledsteel a controlled temperature reduction so as to obtain a preferredmicrostructure of the steel; and further characterized in that (d) theslabs are reheated to a suitable pre-rolling temperature above thetemperature T_(nr) sufficient to transform the quenched surface layer tofine-grained austenite; and (e) the slabs are reduction rolled first ina temperature range above the temperature T_(nr) and then at adecreasing temperature between the temperatures T_(nr) and Ar₃ to obtainfirst a controlled recrystallization of austenite and then a pancakingof the austenite.
 23. The method of claim 22, wherein the controlledtemperature reduction comprises cooling the rolled steel at a rate ofabout 12 C to 20 C per second and to a temperature of about 200 C toabout 350 C below the temperature Ar₃, thereby obtaining in the rolledsteel a preferred microstructure including a substantial portion offine-grained bainite.
 24. The method of claim 22, wherein the controlledtemperature reduction comprises a downstream quench immediately followedby a martensite sustaining cooling, the quench being sufficient toobtain a preferred microstructure including a substantial portion offine-grained martensite, and the sustaining cooling being sufficient tosubstantially maintain and preferably to increase the portion offine-grained martensite obtained in the rolled steel.
 25. The method ofclaim 24, additionally including tempering the rolled steel followingthe sustaining cooling step
 26. The method of any of claims 22-25,wherein the controlled temperature reduction is effected at least inpart by laminar flow cooling.
 27. The method of any of claims 22-26,wherein the upstream quench is applied transversely differentially tocompensate for the transverse surface temperature profile of the caststeel.
 28. The method any of claims 22-27, wherein the reduction rollingcomprises (i) a selected number of flat-pass rolling passes above T_(nr)to achieve a selected flat-pass reduction of the thickness of the steeland recrystallization of the austentite in the steel being rolled, then(ii) a selected number of initial coiler passes performed while thesteel is of coilable thickness and the temperature of the steel is abovethe T_(nr), each said initial coiler pass comprising reducing the steeland then coiling the product in a heated environment at a temperatureabove the Ar₃, then (iii) a selected number of final coiler passesperformed while the temperature of the steel is above the Ar₃, each saidfinal coiler pass comprising reducing the steel and then coiling theproduct in a heated environment at a temperature above the Ar₃.
 29. Themethod defined in claim 22-28, wherein the reduction rolling prior tothe final coiler passes reduces the thickness of the steel by a factorin the order of at least 1.5:1 and wherein the final coiler passesreduce the thickness of the steel by a factor in the order of at least2:1 so that the overall combined reduction of the steel is at leastabout 3:1.
 30. The method defined in any of claims 22-29, for optimizingthe production of steel products in circumstances in which the rollingmill is limited at least in part by coiler furnace capacity and by theinability of the coiler furnaces to coil steel above a maximum coilablethickness; characterized by rolling a maximum-weight slab exceeding thecoiler furnace capacity and severing the slab to obtain an end-productof a target weight and target dimensions, the target weight of theparticular end-product of target dimensions being limited by the coilerfurnace capacity; and further characterized by (a) flat-pass reductionrolling the maximum-weight slab from a pre-rolled thickness to producean interim steel product of a severable thickness exceeding the maximumcoilable thickness; then (b) transversely severing the interim steelproduct into two portions, viz a pre-determined target portion having atarget weight selected to be within the coiler furnace capacity, and aresidual surplus portion; (c) flat-pass rolling the target portion tofurther reduce the target portion from the severable thickness to athickness not exceeding the maximum coilable thickness; (d) coaling thetarget portion an one of the coiler furnaces; (e) flat-pass rolling thesurplus portion from the severable thickness to a desired end-productthickness; then (f) transferring the surplus portion downstream forfurther processing to obtain a surplus end product.
 31. The method asclaimed in claim 30, additionally including, after completion of step(f), (g) flat-pass rolling the target portion to a plate of desiredend-product thickness, then directing the target portion downstream forprocessing as plate end-product.
 32. Apparatus as claimed in claim 1,wherein the temperature reduction station is selectably operable toapply cooling to provide an end-product that is bainite-rich ormartensite-rich.
 33. Apparatus as claimed in claim 32, wherein thetemperature reduction station comprises a downstream quench station andimmediately downstream thereof a controlled cooling facility, whereinboth the quench station and controlled cooling station are operated toproduce martensite-rich steel, and wherein the quench station is idleand the controlled cooling station is operated to produce bainite-richsteel.
 34. Apparatus as claimed in claim 33, wherein the controlledcooling station is a laminar flow cooling station.
 35. Apparatus asclaimed in claim 33 or 34, wherein the quench station comprises RPQquench apparatus.